Various abbreviations that appear in the specification and/or in the drawing figures are defined as follows:
3GPPthird generation partnership projectACKacknowledgeCScyclic shiftDM RSdemodulation reference signaleNBbase station of an EUTRAN/LTE systemE-UTRANevolved UTRAN (also referred to as LTE or 3.9G)IFDMAinterleaved frequency domain multiple accessITUinternational telecommunication unionITU-RITU radiocommunication sectorLTElong term evolutionMU-MIMOmulti-user multiple input multiple outputNACKnegative ACKOCCorthogonal cover codeOFDMAorthogonal frequency division multiple accessPUCCHphysical uplink control channelPUSCHphysical uplink shared channelRel. Aug. 9, 20103GPP Release Aug. 9, 2010RRCradio resource controlSC-FDMAsingle carrier frequency division multiple accessSU-MIMOsingle-user multiple input multiple outputSRIscheduling request indicatorSRSsounding reference signalUEuser equipmentUTRAuniversal mobile telecommunication system terrestrialradio accessUTRANUTRA network
FIG. 1 reproduces FIG. 4.1 of 3GPP TS 36.300, V8.6.0 (2008-09), and shows the overall architecture of the E-UTRAN system. The EUTRAN system includes eNBs, providing the EUTRA user plane and control plane (RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an evolved packet core EPC, more specifically to a Mobility Management Entity MME and to a Serving Gateway SG. The S1 interface supports a many to many relationship between MMEs/SGs and eNBs.
Of particular interest herein are the further releases of 3GPP LTE targeted towards future IMT-A systems, referred to herein for convenience simply as LTE-Advanced (LTE-A). LTE-A is directed toward extending and optimizing the 3GPP LTE Rel 8 radio access technologies to provide higher data rates at very low cost. LTE-A will most likely be part of LTE Rel 10 and is expected to use a mix of local area and wide area optimization techniques to fulfil the ITU-R requirements for IMT-Advanced while keeping the backward compatibility with LTE Rel 8. The 3GPP has opened a study item in LTE Rel 10 for introducing uplink SU-MIMO with 2 or 4 transmission antennas at the UE.
It has been agreed that cyclic shift (CS) separation is to be the primary multiplexing scheme of the DM RSs in LTE-Advanced. CS separation is used already in Rel-8, to multiplex DM RS of different UEs in the case of MU-MIMO.
There have been some proposals to use an orthogonal cover code (OCC) as a complementary multiplexing scheme for that CS separation. Such proposals assert that the number of orthogonal reference signals can be increased, different transmission (TX) bandwidths can be supported for different UEs with MU-MIMO pairing, and the orthogonality between multiplexed DM RSs would be improved. Some competing proposals would use IFDMA as a complementary multiplexing scheme for the CS separation.
The inventors herein view one problem for a SU-MIMO implementation as how to improve the cross correlation properties of different SU-MIMO DM RSs, which are not addressed by the above proposals.
One proposal to randomize the cyclic shifts between two slots of the sub-frame can be seen at publication WO 2008/132073, entitled “Coordinated Cyclic Shift and Sequence Hopping for Zadoff-Chu, Modified Zadoff-Chu, and Block-Wise Spreading Sequences”. The randomization scheme discussed there uses a “clock-table” remapping, but it does not appear to be backwards compatible with Release 8, and it does not appear to specifically account for the possible new extensions such as OCC and IFDMA.